Ferroelectric element and process for producing the same

ABSTRACT

A ferroelectric element comprising a ferroelectric material containing at least two metals, the ferroelectric element having been produced by a process including a sol-gel process in the presence of a thickener and/or an association preventive from aqueous solutions of respective salts of the metals. The ferroelectric element is advantageous in that the handling of starting compounds and the production of the ferroelectric element are easy, the storage stability of the starting compound solution is good, the cost is low, the ferroelectric element can be formed as a thin layer, particles on the surface of the thin layer are fine and dense, and, hence, the surface smoothness is good. The ferroelectric element is excellent also in piezoelectric properties and therefore can be advantageously used as a piezoelectric element in a piezoelectric ink jet head.

TECHNICAL FIELD

[0001] The present invention relates to a ferroelectric element and moreparticularly to a ferroelectric element particularly in the form of athin layer, which is advantageously usable as a piezoelectric element inink jet printers and other devices. The present invention also relatesto a process for producing the ferroelectric element and a ferroelectricprecursor which can be advantageously used in the production of theferroelectric element. Further, the present invention relates to apiezoelectric ink jet head using the above ferroelectric element as apiezoelectric element. The term “piezoelectric” and the term“ferroelectric” used herein are defined as follows. Materials, which,when an external force (a stress from the outside) is applied to acrystal thereof, develop polarization, are called piezoelectrics, and,among the piezoelectrics, those wherein the polarization can be reversedby an external electric field are expressly called ferroelectrics.

BACKGROUND ART

[0002] In recent years, in office automation equipment, such as wordprocessors, personal computers, facsimile machines, various measuringinstruments, such as medical measuring instruments, and other devices,ink jet printers have been extensively used for printing informationfrom these devices at high density. As is well known in the art, in theink jet printer, an ink droplet is ejected from a head section of theprinter and deposited directly onto a recording medium, such asrecording paper, to perform monochrome or color printing. The ink jetprinter has many advantages including that printing can be performed oneven a three-dimensional recording medium, running cost is low sinceplain paper can be used as the recording medium, the head can be easilymounted in the printer, the need to provide the step of transfer,fixation and the like can be eliminated, color printing is easilyperformed, and a sharp color printed image can be provided. The headsection of the ink jet printer can be classified into several typesaccording to the drive system for ejecting ink droplets from the headsection. Among them, a typically and advantageously used one is apiezoelectric ink jet head.

[0003] The piezoelectric ink jet head generally comprises: a pluralityof ink chambers which are disposed at equidistant spaces and function asan ink flow passage and a pressurizing chamber for ejecting an ink; anda nozzle plate mounted on the front end of the ink chambers and equippedwith nozzles, for ejecting an ink, corresponding respectively to the inkchambers; and pressing means for pressurizing an ink within the inkchamber in response to the demand for printing. The pressing meanscomprises a piezoelectric element (known also as “piezo element”), andelectrostrictive effect attained by this piezoelectric element isutilized to create a pressure wave within an ink chamber, filled withink, in the head section, permitting the ink to be ejected through thenozzle in the head section.

[0004] The structure of the piezoelectric ink jet head will be describedin more detail with reference to FIG. 1. An ink jet head 10, a part ofwhich is shown in the drawing, has an ink chamber member 11 comprising aplurality of ink chambers 12 serving as an ink flow passage and apressurizing chamber for ejecting ink. A nozzle plate (not shown)equipped with nozzles disposed so as to correspond respectively to theink chambers 12 is mounted on the front end of the ink chamber 11. Theink pressurized within the ink chamber 12 can be ejected as a dropletthrough the bore of the nozzle. In the ink chamber member 11 shown inthe drawing, pressing means is mounted on the open face of the inkchamber 12. In the example shown in the drawing, the pressing meanscomprises: a diaphragm 15 for creating a change in volume of the inkchamber 12; a piezoelectric element 17 as a driving element fordistorting the diaphragm 15; and an upper electrode 16 and a lowerelectrode 18 which can apply voltage according to need, thepiezoelectric element 17 being sandwiched between the upper electrode 16and the lower electrode 18.

[0005] Ferroelectric elements have been extensively used as thepiezoelectric element for the ink jet head or as an element, forexample, for capacitors, actuators, memories and other elements. Theferroelectric element consists essentially of a ferroelectric or aferroelectric material. Typical ferroelectric materials include an oxideceramic represented by the general formula ABO₃ and having a simpleperovskite structure as shown in FIG. 2 and an oxide ceramic having acomposite perovskite structure represented by the general formula (A₁,A₂, . . . ) (B₁, B₂, . . . )O₃. The term “perovskite structure” usedherein refers to both a simple perovskite structure and a compositeperovskite structure unless otherwise specified. As shown in thedrawing, a ceramic having the above perovskite structure containsmetallic ions A and B in the structure. Examples of more specificferroelectric materials having the above structure include leadzirconate titanate (PZT) represented by the general formula Pb(Zr,Ti)O₃. In particular, ferroelectrics, containing lead (Pb) as one metalcomponent, including PZT are generally known to have large remanence,specific permittivity, and piezoelectric constant and possessesexcellent piezoelectricity and ferroelectricity. In the presentspecification, the ferroelectric material will be described particularlywith reference to PZT.

[0006] A sol-gel process has hitherto been well known as a technique forthe production of PZT, particularly PZT in a thin layer form. Use of thesol-gel process in the production of PZT is advantageous in that ahigh-purity thin layer of PZT can be formed, the composition of theformed thin layer of PZT can reflect the composition of the startingmaterial used, which facilitates the control of the composition and canprovide a thin layer of PZT having high surface smoothness by repetitionof spin coating and firing.

[0007] The production of a thin layer of PZT by the sol-gel process anduse of the thin layer of PZT as a piezoelectric element will bedescribed in more detail. For example, as described in JapaneseUnexamined Patent Publication (Kokai) No. 6-112550, lead acetate isdissolved in acetic acid, and the solution is heated under reflux for 30min. Zirconium tetrabutoxide and titanium tetraisopropoxide are thendissolved in the solution, water and diethylene glycol are addeddropwise thereto, and the mixture is satisfactorily stirred to conducthydrolysis. To the resultant alcohol solution of a PZT precursor isadded polyethylene glycol monomethyl ether in an amount of 10% by weightbased on the PZT precursor, followed by satisfactory stirring. Thus, ahomogeneous sol is prepared. A platinum electrode is formed on a siliconsubstrate, the sol is then spin-coated onto the electrode, and thecoating is heated at about 350° C. Thus, a 2.5 μm-thick, thin,crack-free porous gel layer can be formed.

[0008] Subsequently, the same starting material as the above PZTmaterial is hydrolyzed to form a sol. In this case, however, nopolyethylene glycol monomethyl ether is added. The sol is spin-coatedonto the above thin, porous gel layer to form a coating which is thendried by heating at 400° C. The thin layer thus formed is fired in anoxygen atmosphere for 15 hr. The firing temperature is generally 600 to700° C. Thus, a thin layer of PZT having a perovskite structure can beformed through the above series of steps. The above sol-gel reaction maybe represented by a general formula as shown in FIG. 3 wherein Rrepresents an alkyl group.

[0009] Further, hydrothermal synthesis has hitherto been well known as amethod for the formation of a thin layer of PZT from an aqueous solutionof a main starting compound. The formation of the thin layer of PZT byhydrothermal synthesis will be described. For example, as described inJapanese Unexamined Patent Publication (Kokai) No. 6-112543, 0.2 mol oflead nitrate, 0.104 mol of zirconium oxychloride, and 0.096 mol oftitanium tetrachloride are dissolved in a 2 N aqueous potassiumhydroxide solution. A silicon substrate with a platinum electrodeprovided thereon is immersed in the solution, and the system is heatedin an autoclave at 160° C. for 30 hr. The substrate is taken out of theautoclave and dried at 200° C. for one hr to form a thin layer of PZT ofcubic particles having an average diameter of 5 μm.

[0010] Alternatively, the hydrothermal synthesis may comprise a seedcrystal formation process and a crystal growth process. At the outset,in order to form a seed crystal of PZT, a titanium substrate is immersedin water containing lead hydroxide Pb(OH)₂ and zirconium hydroxideZr(OH)₄, and the system is heated in an autoclave at a temperature of140 to 200° C. This heating results in the formation of a thin layer ofPZT, capable of serving as a seed crystal for subsequent layerformation, on the surface of the titanium substrate. After the formationof the seed crystal, the titanium substrate is immersed in watercontaining Pb(OH)₂, Zr(OH)₄, and titanium hydroxide Ti(OH)₄, and thesystem is heated in an autoclave at a temperature of 80 to 150° C. Theheating results in the formation of a layer of PZT, in a coarse particleform, having a larger thickness than the thin layer of PZT formed aboveon the thin layer of PZT.

[0011] The formation of the thin layer of PZT using the hydrothermalsynthesis has advantages including that the layer thickness can beincreased at a low temperature of 200° C. or below and an additionalstep, which renders the process complicated, is unnecessary since thepiezoelectricity is developed immediately after the layer formation, andthe adhesion to the substrate is excellent.

[0012] The above conventional methods for producing a ferroelectricmaterial, however, involves many problems to be solved. For example, inthe method, described in Japanese Unexamined Patent Publication (Kokai)No. 6-112550, wherein the thin layer of PZT is formed by the sol-gelprocess using a metal alkoxide as a main starting compound, use of thealcohol as a solvent for the PZT precursor poses a problem that theviscosity of the precursor varies depending upon the moisture content ofthe air, leading to the occurrence of uneven properties of the formedthin layer of PZT. Further, in order to avoid the adverse effect of themoisture in the air and, therefore, to avoid the formation ofinsolubilized metal alkoxide, the starting compounds should be mixedtogether in a specific atmosphere, so that the handling of the startingcompounds is not easy. Furthermore, in the sol-gel process, it isdifficult to increase the thickness of the thin layer of PZT.

[0013] This is true of the hydrothermal synthesis. For example, for thethin layer of PZT formed by the hydrothermal synthesis described inJapanese Unexamined Patent Publication (Kokai) No. 6-112543, the averagediameter of PZT particles constituting the thin layer is so large thatthe surface smoothness of the layer is low and it is difficult to formthe upper electrode thereon. Further, the hydrothermal synthesisinvolves problems which include the density of the thin layer being lowdue to coarse PZT particles and that potassium (K) is left in the thinlayer, adversely affecting the properties.

[0014] The present inventors have further made extensive and intensivestudies and, as a result, have found that even use of water instead ofthe alcohol according to the present invention creates problems in somecases.

[0015] As described below in detail, according to the present invention,preferably, three metal salts or metal alkoxides, lead nitrate(Pb(NO₃)₂), zirconium oxynitrate (ZrO(NO₃)₂), and titanium isopropoxide(Ti(O-i-C₃H₇)₄), are used as the starting compound to prepare an aqueousPZT precursor solution. The aqueous PZT precursor solution is coatedonto a predetermined substrate, and the PZT coating is dried and firedto form a thin layer of PZT. In this case, a first possible problem isassociation of lead in the step of drying the PZT precursor coating. Ingeneral, when components, which are different from each other insolubility in water, are mixed together to prepare an aqueous solution,the components contained in the solution associate with each other inthe course of the subsequent step of drying the solution. Thisphenomenon occurs also in the step of drying the PZT precursor coating,and the association of lead is significant. As a result, there is apossibility that a material is significantly precipitated in the form ofcrossed stripes on the surface of the thin layer of PZT. Morespecifically, this is apparent from a microphotograph (magnification:20×) shown in FIG. 4. The creation of the association of lead isconsidered to result in not only the creation of undesired defects onthe surface of the thin layer but also other problems connected with thedefects.

[0016] In PZT ceramics, it is known that, regarding the ratio ofcomponents constituting the PZT ceramics, a Pb:Zr:Ti:O ratio of1:0.53:0.47:3 offers the highest piezoelectric properties and, when theratio deviates from this ratio, the piezoelectric properties are rapidlydeteriorated. Therefore, in the thin layer of PZT thus formed, even whenthe Pb:Zr:Ti:O ratio is the above desired value in the stage of theprecursor, the association of lead created in the course of drying leadsto an undesired variation of the ratio, resulting in the formation of anuneven layer which provides deteriorated piezoelectric properties. Inaddition, this lowers the density of the layer.

[0017] A second possible problem is the creation of defects such ascracks or pinholes. For example, coating of the aqueous PZT precursorsolution onto a substrate by a conventional method, such as dip coatingor spin coating, followed by drying, degreasing, and firing to form athin layer of PZT often creates cracks when the thickness of the thinlayer is 1 μm or more. The creation of cracks could not be avoided eventhough the thin layer of PZT is formed by stacking a plurality ofthinner layers on top of each other or one another. The creation ofcracks in the thin layer of PZT results in lowered density of the layer,making it impossible to form an element, such as an electrode, on thetop of the layer. Therefore, the formed thin layer of PZT cannot beused, for example, as a piezoelectric element of an ink jet head.Further, since the aqueous PZT precursor solution used in this case hasa low viscosity on the order of several centipoises, the coverage percoating is small and, in addition, pinholes and the like are likely tooccur.

[0018] In view of the importance of the problem of the creation ofcracks or pinholes, the present inventors have made an experiment and,as a result, have found that coating of an aqueous PZT precursorsolution having a composition with the Pb:Zr:Ti:O ratio being1:0.53:0.47:3 (the above described preferred ratio) onto a substrate bydip coating followed by drying at 150° C. develops the formation ofprotrusions of a material in a crossed stripe form. After the subsequentfiring at 700° C. for crystallization, the protrusions were present, andno noticeable disappearance of the protrusions was observed. EDX (energydispersive X-ray analysis) has revealed that the material in a stripeform constituting the protrusions has a high lead content. Further, whenan aqueous solution of lead, an aqueous solution of zirconium, and anaqueous solution of titanium were prepared as described above, droppedon a substrate and allowed to stand at room temperature, a crystal wasprecipitated only in the aqueous lead solution.

[0019] The thickness of the thin layer of PZT formed from the aqueousPZT precursor solution is 0.05 μm at the largest per coating step, and,in addition, pinholes were created. Further, when a series of steps ofcoating, drying, degreasing, and firing were repeated ten times, thethickness of the formed thin layer of PZT was 0.5 μm, and, in addition,cracks were created.

DISCLOSURE OF THE INVENTION

[0020] Accordingly, a first object of the present invention is toprovide a ferroelectric element having advantages including that thehandling of starting compounds and the production of the ferroelectricelement are easy, the storage stability of the starting compoundsolution is good, the cost is low, the ferroelectric element can beformed as a thin layer, particles on the surface of the thin layer arefine and dense, and, hence, the surface smoothness is good.

[0021] A second object of the present invention is to provide aferroelectric element which, in addition to the above properties, whenformed as a thin layer, permits the layer thickness to be increased andhas excellent adhesion to the underlying substrate.

[0022] A third object of the present invention is to provide aferroelectric element which, when formed as a thin layer, does notcreate any defect, such as a precipitate of a material in a stripe form,on the surface of the thin layer, and enables the formation of a denseand even thin layer and, in addition, possesses excellent piezoelectricproperties.

[0023] A fourth object of the present invention is to provide aferroelectric element which, when formed as a thin layer, does notcreate any defect, such as a precipitate of a material in a stripe form,on the surface of the thin layer, enables the formation of a dense andeven thin layer, possesses excellent piezoelectric properties and, inaddition, does not create defects, such as cracks or pinholes.

[0024] A fifth object of the present invention is to provide a processfor producing the above excellent ferroelectric element.

[0025] A sixth object of the present invention is to provide aferroelectric precursor which can be advantageously used for theproduction of the above excellent ferroelectric element.

[0026] A seventh object of the present invention is to provide apiezoelectric ink jet head comprising the ferroelectric element of thepresent invention as a piezoelectric element.

[0027] Other objects of the present invention could be easily understoodfrom the following detailed description.

[0028] According to one aspect of the present invention, there isprovided a ferroelectric element comprising a ferroelectric materialcontaining at least two metals, said ferroelectric element having beenproduced in the presence of a thickener and/or an association preventivefrom aqueous solutions of respective salts of the metals. Preferably, asol-gel process is used for the preparation of a solution in theformation of the element.

[0029] In this case, as described above, the ferroelectric materialconsisting essentially of the ferroelectric element of the presentinvention is preferably an oxide ceramic having a simple or a compositeperovskite structure, more preferably lead zirconate titanate (PZT)represented by the general formula Pb(Zr, Ti)O₃. The PZT ceramic ispreferably, but not limited to, one having a Pb:Zr:Ti:O ratio of1:0.53:0.47:3 from the viewpoint of good piezoelectric properties andexcellent other properties. In the present specification, although thepractice of the present invention will be described particularly withreference to the PZT ceramic, the present invention can beadvantageously applied also to other ferroelectric materials.

[0030] In the ferroelectric element according to the present invention,the thickener added to the aqueous ferroelectric precursor solution ispreferably a water-soluble polymeric material which can beheat-decomposed when the temperature exceeds a predetermined temperaturein the formation of the element, particularly a water-soluble polymericmaterial which can be heat-decomposed at the time of degreasing or thelike. Suitable thickeners include, but are no limited to, for example,hydroxyalkyl celluloses with the number of carbon atoms in the alkylgroup being preferably 2 to 4, for example, hydroxyethyl cellulose orhydroxypropyl cellulose, polyethylene oxide, and polyvinyl alcohol.These thickener compounds may be used alone or as a mixture of two ormore.

[0031] In the ferroelectric element of the present invention, theassociation preventive used, either alone or in combination with thethickener, is preferably a water-soluble polyhydric alcohol. Suitablepolyhydric alcohols include, but are not limited to, for example,diethylene glycol, polyethylene glycol, and glycerin. These polyhydricalcohols may be used alone or as a mixture of two or more.

[0032] In practicing the present invention, preferably, the thickenerand/or the association preventive are added to the aqueous ferroelectricprecursor solution to permit a powder of a ferroelectric having acrystal structure identical or similar to the ferroelectric to exist.

[0033] The ferroelectric element of the present invention is preferablya thin layer which has been formed from the above ferroelectricprecursor solution through a solution preparation process using asol-gel process. Although the thin layer generally has a single layerstructure, it may if necessary have a laminate structure of two or morelayers.

[0034] According to one preferred embodiment of the present invention, asubstrate layer having a crystal structure identical or similar to theferroelectric of the element and formed by hydrothermal synthesis fromaqueous solutions of metals necessary for the formation of the substratelayer may be provided as a layer underlying the ferroelectric element ina thin layer form. In this case, the underlying ferroelectric layer, ascompared with the ferroelectric provided on the ferroelectric layer,comprises particles having a larger diameter and has a lower density.

[0035] According to another aspect of the present invention, there isprovided a process for producing a ferroelectric element comprising aferroelectric material containing at least two metals, said processcomprising the steps of:

[0036] mixing aqueous solutions of respective salts of the metalstogether to prepare an aqueous ferroelectric precursor solution;

[0037] adding a thickener and/or an association preventive to theaqueous ferroelectric precursor solution and coating the resultantsolution onto a substrate; and

[0038] drying and firing the coating to crystallize the ferroelectricmaterial.

[0039] According to still another aspect of the present invention, thereis provided a ferroelectric precursor for use as a starting material inthe production of a ferroelectric material comprising at least twometals by a process including a sol-gel process, said ferroelectricprecursor comprising aqueous solutions of the respective salts of themetals and containing a thickener and/or an association preventive.

[0040] According to a further aspect of the present invention, there isprovided an ink jet head comprising a plurality of nozzles for ejectingan ink, ink chambers, for passage and pressurization of the ink,communicating with the nozzles, and pressing means for creating a changein volume of the ink in the ink chamber to eject the ink through thenozzles,

[0041] the pressing means comprising a ferroelectric element as apiezoelectric element, said ferroelectric element comprising aferroelectric material containing at least two metals and having beenproduced by a process including a sol-gel process in the presence of athickener and/or an association preventive from aqueous solutions ofrespective salts of the metals.

[0042] Basically, the ink jet head of the present invention can have thesame construction as the piezoelectric ink jet head commonly used in theart and is not particularly limited so far as the thin layer of theferroelectric is used as the piezoelectric element. Therefore, asuitable ink jet head is, for example, an ink jet head which has beendescribed above with reference to FIG. 1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 is a cross-sectional view showing a principal part of aconventional piezoelectric ink jet head;

[0044]FIG. 2 is a schematic diagram showing the structure of an oxideceramic having a simple perovskite structure;

[0045]FIG. 3 is a flow sheet showing a process for producing leadzirconate titanate (PZT) by the conventional sol-gel process;

[0046]FIG. 4 is a microphotograph showing a material in a crossed stripeform formed on the surface of a thin layer of PZT in the formation ofthe thin layer of PZT by the conventional liquid phase process;

[0047]FIG. 5 is a flow sheet showing one preferred embodiment of theprocess for producing an aqueous PZT precursor solution according to thepresent invention;

[0048]FIG. 6 is a flow sheet showing one preferred embodiment of theprocess for producing a thin layer of PZT using as a starting materialthe aqueous PZT precursor solution prepared according to the process asshown in FIG. 5;

[0049]FIG. 7 is a flow sheet showing the process for producing PZTaccording to the present invention;

[0050]FIG. 8 is a flow sheet showing one preferred embodiment of theprocess for producing a PZT paste, instead of the aqueous PZT precursorsolution, according to the present invention;

[0051]FIG. 9 is a cross-sectional view showing the steps of forming athin, composite PZT layer in sequence according to another preferredembodiment of the present invention;

[0052]FIG. 10 is a graph showing an X-ray diffraction pattern of a PZTpowder prepared using hydroxypropyl cellulose as a thickener accordingto the present invention;

[0053]FIG. 11 is a graph showing an X-ray diffraction pattern of acomparative PZT powder prepared without use of any thickener;

[0054]FIG. 12 is a graph showing an X-ray diffraction pattern of a thinlayer of PZT prepared using hydroxypropyl cellulose as a thickeneraccording to the present invention;

[0055]FIG. 13 is a graph showing an X-ray diffraction pattern of a thinlayer of PZT prepared using polyethylene oxide as a thickener accordingto the present invention;

[0056]FIG. 14 is a graph showing an X-ray diffraction pattern of a thinlayer of PZT prepared using polyvinyl alcohol as a thickener accordingto the present invention.

[0057]FIG. 15 is a microphotograph showing the surface state of a thinlayer of PZT prepared using polyethylene glycol as an associationpreventive according to the present invention; and

[0058]FIG. 16 is a microphotograph showing the surface state of acomparative thin layer of PZT (as a control) prepared using polyethyleneglycol as the association preventive in a reduced amount.

BEST MODE FOR CARRYING OUT THE INVENTION

[0059] Preferred embodiments of the present invention will be describedin more detail particularly with reference to lead zirconate titanate(PZT). An embodiment wherein the ferroelectric element of the presentinvention is used particularly as a piezoelectric element of an ink jethead will be described. However, it should be noted that theferroelectric element can also be advantageously applied to otherdevices.

[0060] The ferroelectric element according to the present inventioncomprises a ferroelectric material containing at least two metals and isproduced by using, as starting materials, aqueous solutions of therespective salts of metals constituting the ferroelectric. Specifically,a thickener and/or an association preventive are added to aqueoussolutions of metal salts selected for the ferroelectric, preferablyaqueous solutions of metal oxides to prepare an aqueous precursorsolution, the aqueous precursor solution is coated, either as such or,if necessary, after preparation of a paste from the aqueous precursorsolution, onto a substrate, and the coating is dried and fired.

[0061] For example, a thin layer of PZT, which is a representativeexample of the ferroelectric element of the present invention, may beformed by preparing an aqueous solution of a salt of lead, an aqueoussolution of a salt of zirconium, and an aqueous solution of a salt oftitanium, mixing these aqueous solutions together to prepare an aqueousPZT precursor solution, coating the aqueous precursor solution onto apredetermined substrate, and then subjecting the coating to treatmentsuch as drying and firing.

[0062] The above series of steps for forming the thin layer of PZT aredescribed in more detail in FIGS. 5 and 6 (flow sheets) and will bedescribed below in sequence. One preferred embodiment of each step willbe described, but it will be understood that variations andmodifications can be effected within the spirit and scope of theinvention.

[0063] For the formation of a thin layer of PZT described herein, threemetal salts, lead nitrate (Pb(NO₃)₂), zirconium oxynitrate (ZrO(NO₃)₂),and titanium isopropoxide (Ti(O-i-C₃H₇)₄), are used as the startingcompound.

[0064] Preparation of Aqueous Titanium Solution

[0065] A predetermined amount of titanium isopropoxide (Ti(O-i-C₃H₇)₄)is dissolved in a 3 N aqueous nitric acid solution, and the solution isstirred in a nitrogen atmosphere to perform mixing. The reason why themixing is performed in a nitrogen atmosphere is that, when the mixing isperformed in air, titanium isopropoxide is insolubilized through areaction with moisture in air to produce TiO₂. The mixing in a nitrogenatmosphere gives a homogeneous aqueous titanium solution through thefollowing reaction

Ti(O-i-C₃H₇)₄+2H₂O→Ti(OH)₄+4C₃H₇OH

[0066] Preparation of Aqueous Zirconium Solution

[0067] A predetermined amount of zirconium oxynitrate is dissolved inpure water, and the solution is stirred to perform mixing. The mixinggives a homogeneous aqueous zirconium solution through the followingreaction.

ZrO(NO₃)₂+2H₂O→Zr(OH)₄+2HNO₃

[0068] Mixing

[0069] The aqueous titanium solution and the aqueous zirconium solutionprepared above are mixed together while stirring to prepare ahomogeneous solution.

[0070] Preparation of Aqueous Lead Solution

[0071] A predetermined amount of lead nitrate is dissolved in purewater, and the solution is stirred to perform mixing. The mixing gives ahomogeneous aqueous lead solution through the following reaction.

Pb(NO₃)₂+2H₂O→Pb(OH)₂+2HNO₃

[0072] In this case, for example, another salt, such as lead acetate,may be used instead of lead nitrate.

[0073] Preparation of Aqueous PZT Precursor Solution

[0074] The aqueous lead solution thus prepared is mixed with the mixedaqueous solution, containing titanium and zirconium, prepared above toprepare a homogeneous aqueous PZT precursor solution. The concentrationof the aqueous PZT precursor solution is not particularly limited andmay be widely varied by taking desired results, film forming conditionsand other various conditions, into consideration. The present inventorshave found that the concentration of PZT in the aqueous PZT precursorsolution is preferably about 5 to 20% by weight, more preferably about10% by weight. The aqueous PZT precursor solution, as compared with theconventional alcohol solution of the PZT precursor, is easy to handleand is inexpensive. Thus, the series of steps shown in FIG. 5 arecompleted. Subsequently, the step of forming a thin layer of PZT (seeFIG. 6) is carried out.

[0075] Addition of Thickener and/or Association Preventive

[0076] A thickener and an association preventive are added, either aloneor in combination, to the aqueous PZT precursor solution prepared aboveaccording to the present invention. These additives may be added in anyorder. The thickener and the association preventive will be described inmore detail below.

[0077] After the addition of the necessary additive(s) has beencompleted, the mixture is thoroughly stirred. Thus, a coating solutionfor a thin layer of PZT is prepared. If desired, the coating solutionmay be treated to prepare a paste.

[0078] Coating (Formation of Coating)

[0079] The coating solution or the paste prepared above is coated on apredetermined substrate to form a desired pattern. The coating may beperformed by a conventional method, and examples of coating methodsusable herein include spin coating, dip coating, and screen printing. Anoptimal coating method may be selected according to the type ofcontemplated thin layer of PZT. Likewise, the coverage, that is, thethickness of the coating formed, may be suitably selected according tovarious factors. For example, a submicron layer thickness after dryingsuffices for use of the resultant thin layer of PZT as a memory, acapacitor and the like. On the other hand, a layer thickness on theorder of several tens of μm suffices for use of the thin layer of PZT asan actuator.

[0080] Driving

[0081] Subsequently, the coating thus formed is dried to cure thecoating and, at the same time, to remove excess water or the like byevaporation. The drying temperature and time may be widely varied. Ingeneral, however, the drying is performe d at about 100 to 200° C. forabout 5 to 30 min. For example, drying at 150° C. for 10 min may beadopted. Defects, such as cracks, are not created in the coating in thecourse of the step of drying. Further, the step of degreasing and otherconventional treatment steps (not shown) may be interposed between thestep of drying and the step of firing.

[0082] Firing

[0083] Finally, the dried coating is fired. As with the step of dryingdescribed above, the step of firing may be carried out by any methodcommonly used in the art. The firing temperature is preferably about 500to 900° C., more preferably about 700 to 800° C. In general, firing at atemperature around 500° C. can offer the desired results. The firingtime may be widely varied according to the relationship thereof with thefiring temperature or the like. In general, however, it is in the rangeof about 1 to 60 min. A thin layer of PZT, which has high density and isvery high compact, is formed as a result of the firing. As describedabove, when the firing temperature is low, a problem of scattering oflead (Pb) during the firing can be avoided. Therefore, a high-qualitythin layer of PZT can be obtained while maintaining the same composition(Pb:Zr:Ti:O ratio) as that of the PZT precursor provided above.

[0084] The state in the course of the formation of the thin layer of PZTis shown in FIG. 7. From comparison of FIG. 7 with FIG. 3 describedabove, it will be understood that the structure of the gel according tothe present invention is distinguished from that of the conventionalgel.

[0085] In the formation of the above thin layer of PZT, the Pb:Zr:Ti:Oratio in the PZT precursor may be widely varied according to the desiredresults. In order to provide the best piezoelectric properties, asdescribed above, it is recommended that Pb:Zr:Ti:O be 1:0.53:0.47:3. Inthe present invention, when the above composition ratio is adopted inthe PZT precursor, the ratio may be reproduced as it is in the thinlayer of PZT, enabling a thin layer of PZT, which is homogeneous, hashigh density and possesses excellent piezoelectric properties, to beeasily provided.

[0086] Regarding the creation of defects, such as cracks or pinholes,described above in connection with the prior art, according to thepresent invention, addition of a PZT powder in addition to the thickenerand/or the association preventive in the stage of preparing the aqueousPZT precursor solution can effectively prevent the creation of thedefects. In this case, preferably, the PZT precursor is in the form of apaste rather than an aqueous solution.

[0087]FIG. 8 shows a flow sheet of this preferred embodiment. Theprocedure described above with reference to FIG. 5 may be repeated up tothe step at which the aqueous PZT precursor solution is prepared. Next,as described above with reference to FIG. 6, the thickener and/or theassociation preventive are added. The PZT powder is added simultaneouslywith, before or after the addition of these additives. The order ofadding the above additives may be properly varied according to thedesired results and other factors. PZT powders usable herein will bedescribed in more detail below.

[0088] After the addition of the necessary additives has been completed,the mixture is placed in a milling device, for example, a planetary ballmill, followed by milling for several minutes. If necessary, theaddition of the PZT powder may be carried out in the step of milling.Thus, a homogeneous paste can be obtained which is then coatable byscreen printing or the like. Preferably, after vacuum deaeration forseveral min by means of a rotary pump, the paste is coated. The steps ofcoating, drying, and firing may be carried out in the same manner asdescribed above with reference to FIG. 6.

[0089] In the practice of the present invention, the aqueous PZTprecursor solution may contain a thickener. Preferably, the thickenermay be added after the preparation of the aqueous precursor solution.Functions of the thickener include, for example, an improvement incrystallinity of the thin layer of PZT and, when the PZT powder is alsoused, homogeneous dispersion of the PZT powder. The thickener is oftencalled a binder from the viewpoint of the function. A suitable thickeneris a water-soluble polymeric material which can be heat-decomposed whenthe temperature exceeds a predetermined temperature in the formation ofthe element (that is, at the time of firing). Examples of thickenerswhich can be advantageously used in the present invention include, butare not limited to, hydroxyalkyl celluloses with the number of carbonatoms in the alkyl group being preferably 2 to 4 (for example,hydroxypropyl cellulose), polyethylene oxide, and polyvinyl alcohol.These thickener compounds may be used alone or as a mixture of two ormore. The amount of the thickener used may be widely varied according tofactors such as the desired effects. In general, however, the amount isabout 0.1 to 50% by weight based on the total amount of the aqueous PZTprecursor solution. Preferably, when the thickener used is hydroxypropylcellulose, the amount thereof is generally 0.5 to 10% by weight based onthe total amount of the aqueous PZT precursor solution. When the purposeof adding the thickener is to homogeneously disperse the PZT powder, theamount of the thickener added may be relatively large.

[0090] In the practice of the present invention, the associationpreventive is used, either in combination with the thickener orindependently of the thickener, in the aqueous PZT precursor solution.The association preventive, when contained in the aqueous precursorsolution, can be coordinated to a lead (Pb) element contained in theaqueous solution to effectively prevent the association of lead elementswith each other which has been the problem of the prior art. A suitableassociation preventive is a water-soluble polyhydric alcohol, andexamples of polyhydric alcohols, which can be advantageously used in thepresent invention, include, but are not limited to, diethylene glycol,polyethylene glycol, and glycerin. These association preventives may beused either alone or in combination. The amount of the associationpreventive used may be widely varied according to factors such as thedesired effects. In general, however, the amount is about 5 to 20% byweight based on the total amount of the aqueous PZT precursor solution.

[0091] In the present invention, it is possible to attain not only theeffects inherent in the thickener and the association preventive by theaddition of these additives but also the effect, described above, by thedissolution of the PZT precursor in water instead of an alcohol. Thatis, the problem of the variation in viscosity of the precursor due tothe moisture in the air, which is experienced in the use of an alcoholas the solvent, can be avoided. Therefore, the precursor can be easilyused in a very stable state. Practice of the sol-gel process using thisprecursor results in the formation of a thin layer of PZT which is denseand possesses excellent piezoelectric properties.

[0092] In the practice of the present invention, when there is a fear ofcracks or pinholes being created due to the use of the aqueous PZTprecursor solution, or even though the above fear does not exist,addition of another PZT powder having a crystal structure identical orsimilar to the PZT, in addition to the addition of the thickener and/orthe association preventive, to the aqueous PZT precursor solution ispreferred. The PZT powder added in this case can simultaneously exhibitvarious noteworthy functions. For example, the PZT powder, when the PZTcoating after drying is fired, can reduce the shrinkage of the coatingby the volume of the powder added, leading to the prevention ofcracking. Further, addition of the PZT powder can increase the coverageof PZT per coating step and hence can increase the thickness of the thinlayer of PZT. The amount of the PZT powder added may widely varyaccording to factors such as the desired effects. In general, however,the amount is about 5 to 20% by weight based on the total amount of theaqueous PZT precursor solution. The above effects have been describedparticularly with reference to the addition of a PZT powder. Also in theproduction of ferroelectric elements other than PZT, addition of acorresponding ferroelectric powder can provide effects comparablefavorably with the above effects.

[0093] Further, in the practice of the present invention, as describedabove, addition of the thickener in combination with the PZT powder ispreferred. Addition of the thickener enables the viscosity of the PZTprecursor paste to be controlled and hence can control the thickness ofthe resultant thin layer of PZT. Addition of the additives followed byagitating operation, such as milling, permits the mixed PZT powder to behomogeneously dispersed. Further, settling of the PZT powder can beeffectively prevented. Furthermore, the addition of the thickener canimprove the wettability of the substrate by the PZT paste and theadhesion of the thin layer of PZT after firing to the substrate. Whenthe thickener is used in combination with the PZT powder, the amount ofthe thickener used may vary widely depending upon the desired viscosity,the desired effects and other factors in the paste. In general, however,the amount is about 1 to 30% by weight based on the total amount of theaqueous PZT precursor solution.

[0094] The addition of additional additives brings the PZT precursor asthe starting material in the form of a solution to a paste. Theresultant PZT paste may be coated by the above-described methods, suchas spin coating and dip coating. When the viscosity is taken intoconsideration, however, coating may be advantageously performed byscreen printing and other methods commonly used in the field of coatingformation. Use of the screen printing enables the PZT coating to beformed in a desired pattern and, at the same time, can facilitateincreasing the coating thickness.

[0095] Thus, addition of the PZT powder, in addition to the thickenerand/or the association preventive, to the aqueous PZT precursor canprovide a PZT paste which has excellent storage stability and high PZTconcentration. Use of this paste results in the formation of a thinlayer of PZT which is free from defects, such as pinholes and cracks,and possesses excellent piezoelectric properties.

[0096] According to another preferred embodiment of the presentinvention, a ferroelectric element, such as a thin layer of PZT, isproduced by combining hydrothermal synthesis with the sol-gel process.In this case, the hydrothermal synthesis is basically used for theformation of a first layer (that is, a ferroelectric substrate layer) ofthe ferroelectric element by the conventional method. On the other hand,the sol-gel process is basically used for the formation of a secondlayer (an upper ferroelectric layer) of the ferroelectric element usingas a starting material an aqueous ferroelectric precursor solutioncontaining at least a thickener and/or an association preventive andoptionally a ferroelectric powder.

[0097] That is, the ferroelectric element according to this preferredembodiment is characterized in that the upper ferroelectric layerconstituting the main part of the ferroelectric element has, on theunderside thereof, the ferroelectric substrate layer which has a crystalstructure identical or similar to the ferroelectric materialconstituting the upper ferroelectric layer and has been formed, fromaqueous solutions of metals necessary for the formation of theferroelectric substrate layer, by hydrothermal synthesis. In the abovecomposite ferroelectric element, preferably, the ferroelectric materialfor the ferroelectric substrate layer, as compared with theferroelectric material for the upper ferroelectric layer, is constitutedby particles having a larger diameter and has lower density.

[0098] Preferably, the above composite ferroelectric element, forexample, a composite thin PZT layer, may be produced, for example, by aprocess comprising steps shown in sequence in FIG. 9.

[0099] At the outset, as shown in the step A, a substrate 1 is provided.For the substrate, the type, shape and the like may widely varydepending upon the contemplated application of the composite thin PZTlayer and other factors. Suitable substrates include, for example,conventional substrates, such as ceramic substrates, for example,silicon substrates and titanium substrates, and glass substrates. Thesubstrate may have thereon a layer, such as an insulating layer, wiring,or an electrode. In the present embodiment, a titanium substrate isused.

[0100] Next, in the step B, a seed crystal layer 2 of PZT is formed onthe titanium substrate 1. The seed crystal layer 2 may be formed, forexample, by immersing the titanium substrate 1 in water containing leadhydroxide (Pb(OH)₂) and zirconium hydroxide (Zr(OH)₄) and heating thesystem in an autoclave at a temperature of 140 to 200° C.

[0101] After the formation of the seed crystal layer 2 has beencompleted, in the step C, a first PZT layer (substrate layer) 3 isfurther formed on the seed crystal layer 2. This PZT layer 3 may beformed, for example, by immersing the titanium substrate 1, with theseed crystal layer 2 formed thereon, in water containing Pb(OH)₂,Zr(OH)₄, and titanium hydroxide (Ti(OH)₄) and heating the system in anautoclave at a temperature of 80 to 150° C. As a result, on the seedcrystal layer 2 formed above is formed a PZT layer 3, as shown in thedrawing, which, as compared with the seed crystal layer 2, isconstituted by coarser particles and has larger layer thickness. The PZTlayer 3 is advantageous in that an increase in the layer thickness ispossible at a low temperature and, in addition, the PZT layer 3 candevelop piezoelectricity immediately after the layer formation and hasgood adhesion to the titanium substrate 1.

[0102] Finally, in the step D, a second PZT layer 4 is formed using theaqueous PZT precursor solution or the paste according to the presentinvention. Disposition of the PZT layer 4 on the first PZT layer 3 cansmoothen the roughened surface of the underlying PZT layer 3. Since thesurface of the composite thin PZT layer is smooth, an electrode and thelike may be easily formed thereon with high reliability. In the case ofthe above two-layer structure, the creation of defects, such as cracksor pinholes, can be prevented in the composite thin PZT layer.

[0103] The ferroelectric element according to the present invention canbe advantageously used for constituting pressing means in apiezoelectric ink jet head, that is, an ink jet head comprising: aplurality of nozzles for ejecting an ink; an ink chamber, communicatingwith the nozzles, for flow and pressurization of the ink; and pressingmeans for creating a change in volume of the ink in the ink chamber toeject the ink through the nozzle.

[0104] As described above, the ink jet head per se can have the sameconstruction as the piezoelectric ink jet head commonly used in the art.The ink jet head of the present invention will be described again withreference to FIG. 1.

[0105] An ink chamber 12 in an ink jet head 10 is provided in apredetermined pattern in an ink chamber member 11. The ink chambermember 11 may be made of various materials according to factors such asthe method for forming the ink chamber 12. One embodiment of the inkchamber member 11 will be described. For example, glass, a plasticmaterial, for example, a polyester resin (such as PET), an acrylic resin(such as PMMA), quartz, or other substrate is provided, and a resinmaterial may be then patterned onto the substrate by photolithography oranother method to form a groove corresponding to the ink chamber.

[0106] In the ink chamber member 11 shown in the drawing, a diaphragm 15for creating a change in volume of the ink chamber 12 (one constituentelement of pressing means referred to in the present invention) isprovided on the open face of the ink chamber 12. The diaphragm 15 may beintegrally molded with the ink chamber member 11 at the time offormation of the ink chamber member 11. Otherwise, it may be bonded tothe ink chamber member 11 with the aid of an adhesive. Suitablematerials for the diaphragm 15 include, for example, ceramics havinghigh rigidity, for example, ZrO₂. The remaining elements of the pressingmeans are attached so as to abut against the diaphragm 15.

[0107] In the case of the embodiment shown in the drawing, the remainingelements of the pressing means are a piezoelectric element 17 as a drivefor distorting the diaphragm 15 and an upper electrode 16 and a lowerelectrode 18 which sandwich the piezoelectric element 17 therebetweenand, when necessary, can apply a voltage. The piezoelectric element 17may be formed by the above process according to the present invention.The electrodes 16 and 18 each may be formed in a desired pattern by aconventional method, for example, sputtering, screen printing, or vapordeposition.

[0108] The ink jet head shown in the drawing may be operated as follows.An ink is fed into the ink chamber 12 through an ink feed port (notshown) of the head 10 to fill the ink chamber 12 with the ink. In thisstate, application of a voltage across the electrodes 16 and 18 createsa displacement of the piezoelectric element 17 sandwiched between theseelectrodes due to the piezoelectric properties, permitting the diaphragm15 disposed so as to abut against the piezoelectric element 17 to bepushed out towards the ink chamber 12. This reduces the volume of theink filled into the ink chamber 12, leading to ejection of the ink in adot form in an amount corresponding to the reduction of the volumethrough a nozzle (not shown) communicating with the ink chamber 12.

EXAMPLES

[0109] The present invention will be described in more detail withreference to the following examples. It should be understood that thepresent invention is not limited to these examples only.

Example 1

[0110] 0.014 mol of titanium tetraisopropoxide was dissolved in 30 g ofa 3 N aqueous nitric acid solution to prepare an aqueous titaniumsolution. Separately, 0.016 mol of zirconium oxynitrate was dissolved in20 g of pure water to prepare an aqueous zirconium solution. Further,0.03 mol of lead nitrate was dissolved in 20 g of pure water to preparean aqueous lead solution.

[0111] The three aqueous solutions thus prepared were mixed together toprepare an aqueous PZT precursor solution. In this connection, it shouldbe noted that the ratio of Zr to Ti in PZT is closely related to theferroelectricity. Therefore, in the preparation of the aqueous solutionsfollowed by mixing of these aqueous solutions, this has been taken intoconsideration, and the preparation and mixing were performed so that thefinally formed thin layer of PZT had a desired composition ratio(Pb:Zr:Ti:O=1:0.53:0.47:3).

[0112] At the outset, the aqueous titanium solution and the aqueouszirconium solution were thoroughly mixed together at room temperaturewhile stirring. After the completion of the stirring, the mixture wasmixed with the aqueous lead solution. The resultant mixture wasthoroughly stirred at room temperature and then heated under reflux at atemperature of 100° C. or above to conduct hydrolysis. Metal ionscontained in each aqueous solution, when the aqueous solutions weremixed together at room temperature, merely formed corresponding metalhydroxides, and the degree of bonding among the metal ions was small.Subsequent reflux and hydrolysis, however, permit the metal ions, thatis, titanium, zirconium, and lead ions, to be bonded to one anotherthrough oxygen. In this case, a Pb—O—Ti—O—Zr bond is formed to bring thesystem to a sol. In the present invention, the aqueous solution in thissol state is expressly called an “aqueous PZT precursor solution.”

[0113] After the temperature of the aqueous PZT precursor solution wasreturned to room temperature, water and a small amount (about 0.5% byweight) of hydroxypropyl cellulose were added thereto, followed bythorough mixing with stirring at room temperature for about one hour. Asa result, a transparent, homogeneous solution was obtained which wasneither cloudy nor contained any precipitate.

[0114] In this connection, it was confirmed that hydroxypropyl celluloseadded to the aqueous PZT precursor solution has various effects.Specifically, hydroxypropyl cellulose increases the viscosity of theaqueous PZT precursor solution having relatively low viscosity and, whenthe aqueous PZT precursor solution is coated on the substrate in thestep of coating, enables the formation of a coating having largethickness. Not only has hydroxypropyl cellulose the effect of increasingthe viscosity, but also the effect of strengthening the Pb—O—Ti—O—Zrbond and the effect of reducing the distance among the metal ions.Although the reason why these effects can be attained has not been fullyelucidated yet, according to studies conducted by the present inventors,it is believed that the metal ions and oxygen are intertwined withhydroxypropyl cellulose as the backbone, leading to the above effects.As a result, the firing temperature in the layer formation can belowered, resulting in improved crystallinity of the thin layer.

[0115] The results of an experiment conducted by the present inventorsto confirming the above excellent effects of hydroxypropyl cellulosewill be described.

[0116] The aqueous PZT precursor solution prepared as described abovewas placed in a crucible and fired at a predetermined temperature for agiven period of time. As a result, a light-yellow PZT powder (sample 1)was obtained. For comparison, a light-yellow PZT powder (sample 2) wasprepared in the same manner as described above, except thathydroxypropyl cellulose (thickener) was not used. For the samples, theX-ray diffraction pattern was measured. The results were as shown inFIG. 10 (sample 1) and FIG. 11 (sample 2).

[0117] In the X-ray diffraction patterns shown in FIGS. 10 and 11, apeak around an X-ray diffraction angle 2θ=29° is one, derived from apyrochlore layer which appears when the crystallization of PZT isunsatisfactory. The crystallinity of PZT can be quantitativelydetermined by comparing the intensity of this peak with the peakintensity of the (101) face having the highest diffraction intensityamong the PZT diffraction peaks. For sample 1 (FIG. 10) which is anexample of the present invention, the proportion of the pyrochlore layerwas 1.3%, whereas for sample 2 (FIG. 11) which is a comparative example,the proportion of the pyrochlore layer was 11.6%. This indicates thataddition of hydroxypropyl cellulose as the thickener can provide a morecomplete PZT crystal.

Example 2

[0118] A thin layer of PZT was formed from the aqueous PZT precursorsolution prepared in Example 1, and the thin layer of PZT wasincorporated as a piezoelectric element into a piezoelectric ink jethead having a structure shown in FIG. 1.

[0119] A diaphragm of a thin ceramic sheet was prepared, and a platinumelectrode was formed in a predetermined area by sputtering. Further, theaqueous PZT precursor solution prepared in Example 1 was spin-coated at2000 rpm onto the platinum electrode. The PZT coating on the platinumelectrode was dried at 150° C. for 10 min and further pre-fired at 550°C. for one hr. As a result of the pre-firing, hydroxypropyl cellulosecontained in the PZT coating was decomposed and scattered.

[0120] The PZT coating on the platinum electrode was then fired at 700°C. for one min. This allowed the crystallization of PZT to proceed and athin layer of PZT having a perovskite structure was obtained. This thinlayer of PZT had good adhesion to the platinum electrode and thediaphragm and further exhibited satisfactory adhesion in the sputteringof an additional platinum electrode on the thin layer of PZT. Further,since a relatively low temperature can be applied to the firing of thethin film of PZT, the range of selection of the underlying diaphragm canbe broadened.

[0121]FIG. 12 shows an X-ray diffraction pattern of the thin layer ofPZT thus formed. In the X-ray diffraction pattern, no diffraction peakother than peaks derived from PZT is observed, indicating that the layerformed on the platinum electrode is a thin layer of PZT.

Example 3

[0122] The procedure of Example 2 was repeated, except that, in thepreparation of the aqueous PZT precursor solution in the same manner asin Example 1, polyethylene oxide (sample 3) and polyvinyl alcohol(sample 4) were used as a thickener instead of hydroxypropyl cellulose.As a result, satisfactory thin layers of PZT comparable with the thinlayer of PZT formed in Example 2 could be formed.

[0123] The X-ray diffraction patterns of the thin layers of PZT thusformed were as shown in FIG. 13 (sample 3) and FIG. 14 (sample 4). Thethickness of the thin layers of PZT was so small that, in these X-raydiffraction patterns, peaks derived from the platinum electrode could beobserved around diffraction angles, 2θ, of 40°, 47°, and 67°. Peaksother than the peaks derived from platinum were only those derived fromPZT, indicating that the formed layer was a thin layer of PZT.

Example 4

[0124] 0.014 mol of titanium tetrachloride was dissolved in aqueousammonia, and the resultant titanium hydroxide was collected byfiltration and washed. The titanium hydroxide was dissolved in a 3 Naqueous nitric acid solution to prepare an aqueous titanium solution.Separately, 0.016 mol of zirconium oxynitrate was dissolved in 20 g ofpure water to prepare an aqueous zirconium solution. Further, 0.03 molof lead nitrate was dissolved in 20 g of pure water to prepare anaqueous lead solution.

[0125] The three aqueous solutions thus prepared were mixed together toprepare an aqueous PZT precursor solution. At the outset, the aqueoustitanium solution and the aqueous zirconium solution were thoroughlymixed together at room temperature while stirring. After the completionof the stirring, the mixture was mixed with the aqueous lead solution.The resultant mixture was thoroughly stirred at room temperature andthen heated under reflux at a temperature of 100° C. or above to conducthydrolysis. After the temperature of the resultant aqueous PZT precursorsolution was returned to room temperature, water and a small amount(about 0.5% by weight) of hydroxypropyl cellulose were added thereto,followed by thorough mixing with stirring at room temperature for aboutone hr. As a result, a transparent, homogeneous solution was obtainedwhich was neither cloudy nor contained any precipitate.

Example 5

[0126] A thin layer of PZT was formed from the aqueous PZT precursorsolution prepared in Example 4, and the thin layer of PZT wasincorporated as a piezoelectric element into a piezoelectric ink jethead having a structure shown in FIG. 1.

[0127] A diaphragm of a thin ceramic sheet was prepared, and a platinumelectrode was formed in a predetermined area by sputtering. Further, theaqueous PZT precursor solution prepared in Example 4 was spin-coated at2000 rpm onto the platinum electrode. The PZT coating on the platinumelectrode was dried at 150° C. for 10 min and further pre-fired at 500°C. for one hr. As a result of the pre-firing, hydroxypropyl cellulosecontained in the PZT coating was decomposed and scattered.

[0128] The PZT coating on the platinum electrode was then fired at 650°C. for one hr. This allowed the crystallization of PZT to proceed and athin layer of PZT having a perovskite structure was obtained. This thinlayer of PZT had good adhesion to the platinum electrode and thediaphragm and further exhibited satisfactory adhesion in the sputteringof an additional platinum electrode on the thin layer of PZT.

Example 6

[0129] In this example, lead nitrate (Pb(NO₃)₂), zirconium oxynitrate(ZrO(NO₃)₂), and titanium isopropoxide (Ti(O-i-C₃H₇)₄) were used asstarting compounds.

[0130] At the outset, 4.03 g of titanium isopropoxide was dissolved inand mixed with 23.3 g of a 2.8 N aqueous nitric acid solution whilestirring in a nitrogen atmosphere to give an aqueous titanium solution.Separately, 4.27 g of zirconium oxynitrate was dissolved in and mixedwith 18 g of pure water while stirring to give an aqueous zirconiumsolution. After the preparation of the aqueous titanium solution and theaqueous zirconium solution, these aqueous solutions were mixed togetherwhile stirring to prepare a homogeneous solution. Separately, 10 g oflead nitrate was dissolved in and mixed with 32 g of pure water whilestirring. The resultant aqueous lead solution was then mixed with themixed aqueous solution containing titanium and zirconium while stirring.Finally, 10% by weight, based on the resultant mixture, of polyethyleneglycol (weight average molecular weight=200) was added to the mixture.Thus, a homogeneous aqueous PZT precursor solution was prepared.

[0131] In the aqueous PZT precursor solution prepared above, the PZTconcentration was 9% by weight, and the composition (molar ratio ofmetal elements contained) was Pb:Zr:Ti=1:0.53:0.47. Among a number ofPZTs, the PZT having this molar ratio offered the maximum valuesrespectively for the permittivity, the piezoelectric constant and thelike.

[0132] The aqueous PZT precursor solution prepared above was thenspin-coated on a Pt/Ti/Si substrate, and the coating was dried at 150°C. for 10 min, pre-fired at 500° C. for 60 min, and fired at 800° C. for60 min. As a result, a dense thin layer of PZT having a thickness of 100nm was formed. The thin layer of PZT was examined, and, as a result,precipitation of a material in a stripe form on the surface of the thinlayer was not observed.

[0133] Further, in order to evaluate a change in the aqueous PZTprecursor solution with the elapse of time, the aqueous PZT precursorsolution was allowed to stand in air at room temperature for one month.As a result, no change occurred. Since polyethylene glycol used as theassociation preventive in this example has already been found, by TG-DTA(thermogravimetry-differential thermal analysis), to be a material whichcould be degreased at 300° C., it is considered that degreasing could befully achieved in the step of pre-firing.

Example 7

[0134] The procedure of Example 6 was repeated, except that, in thepreparation of the aqueous lead solution, lead acetate, instead of leadnitrate, was used in the same amount. As a result, a satisfactory thinlayer of PZT comparable with the thin layer of PZT formed in Example 6was formed.

Example 8

[0135] In order to evaluate the influence of an association preventiveon lead (Pb) in an aqueous PZT precursor solution, the procedure ofExample 6 was repeated, except that the amount of polyethylene glycolused as the association preventive was reduced from 10% by weight to 5%by weight. As a result, unlike Example 6 (amount of associationpreventive used: 10% by weight) wherein no association of lead wasobserved, in this example (amount of association preventive used: 5% byweight), the association of lead was significant.

[0136] The above results indicate that, since polyethylene glycol usedhere has an average molecular weight of 200 and a monomolecular weightof 63, a large amount of tetramer exists. In this case, regarding thelength of the molecule, Pb²⁺ is 1.2 Å, while polyethylene glycol in atetramer form is several tens of Å. That is, the polyethylene glycolmolecule is considerably longer than Pb²⁺. It is considered that severalmolecules of Pb²⁺ are coordinated to one molecule of polyethyleneglycol, preventing the association of Pb.

Example 9

[0137] The procedure of Example 6 was repeated, except that glycerin wasused instead of polyethylene glycol as the association preventive. As aresult, a 100 nm-thick, dense, thin layer of PZT free from precipitationof a material in a stripe form was formed.

[0138] Further, the procedure of Example 8 was repeated. The results ofevaluation indicate that, although there is a slight difference, as withExample 6, the association of lead could be effectively prevented.

Example 10

[0139] The procedure of Example 6 was repeated, except that diethyleneglycol was used instead of polyethylene glycol as the associationpreventive. As a result, a 100 nm-thick, dense, thin layer of PZT freefrom precipitation of a material in a stripe form was formed.

[0140] Further, the procedure of Example 8 was repeated. The results ofevaluation indicate that, although there is a slight difference, as withExample 6, the association of lead could be effectively prevented.

Example 11

[0141] In this example, lead nitrate (Pb(NO₃)₂), zirconium oxynitrate(ZrO(NO₃)₂), and titanium isopropoxide (Ti(O-i-C₃H₇)₄), were used asstarting compounds.

[0142] At the outset, 4.03 g of titanium isopropoxide was dissolved inand mixed with 23.3 g of a 2.8 N aqueous nitric acid solution whilestirring in a nitrogen atmosphere to give an aqueous titanium solution.Separately, 4.27 g of zirconium oxynitrate was dissolved in and mixedwith 18 g of pure water while stirring to give an aqueous zirconiumsolution. After the preparation of the aqueous titanium solution and theaqueous zirconium solution, these aqueous solutions were mixed togetherwhile stirring to prepare a homogeneous solution. Separately, 10 g oflead nitrate was dissolved in and mixed with 32 g of pure water whilestirring. The resultant aqueous lead solution was then mixed with themixed aqueous solution containing titanium and zirconium while stirring.Finally, 10% by weight, based on the resultant mixture, of polyethyleneglycol (weight average molecular weight=200) was added to the mixture.Thus, a homogeneous aqueous PZT precursor solution was prepared.

[0143] In the aqueous PZT precursor solution prepared above, the PZTconcentration was 9% by weight, and the composition (molar ratio ofmetal elements contained) was Pb:Zr:Ti=1:0.53:0.47. Among a number ofPZTs, the PZT having this molar ratio offered the maximum valuesrespectively for the permittivity, the piezoelectric constant and thelike.

[0144] To the aqueous PZT precursor solution were added a PZT powderhaving the same composition as the PZT and hydroxypropyl cellulose (as abinder) each in an amount of 10% by weight based on the aqueous PZTprecursor solution. The mixture was milled by means of a planetary ballmill for 3 min to prepare a PZT precursor paste.

[0145] The PZT precursor paste thus prepared was coated on a Pt/Ti/Sisubstrate by using a metal mask and a doctor blade, and the coating wasdried at 150° C. for 10 min, degreased at 500° C. for 60 min, and firedat 800° C. for 60 min. As a result, a 3 μm-thick, dense, thin layer ofPZT was formed. The thin layer of PZT was examined and found to be freefrom cracking.

[0146] Thin layers of PZT having varied thicknesses (2 to 4 μm) wereformed in the same manner as described above, except that the amount ofpolyethylene glycol (used in an amount of 10% by weight in the abovecase) and the amount of the PZT powder (used in an amount of 10% byweight in the above case) were varied. As a result, as with the abovecase, crack-free, dense, thin layers of PZT could be formed.

[0147]FIG. 15 is a microphotograph (magnification: 20×) showing thesurface appearance of the thin layer of PZT (thickness 3 μm) thusformed. From this microphotograph, it is apparent that the surface ofthe thin layer of PZT is free from cracks, pinholes, and material in astripe form.

[0148] Further, in order to evaluate a change in the aqueous PZTprecursor solution with the elapse of time, the aqueous PZT precursorsolution was allowed to stand in the air at room temperature for onemonth. As a result, no change occurred. Since polyethylene glycol usedas the association preventive in this example has already been found, byTG-DTA, to be a material which could be degreased at 300° C., it isconsidered that degreasing could be fully achieved in the step ofdegreasing.

Example 12

[0149] In order to evaluate the influence of an association preventiveon lead (Pb) in an aqueous PZT precursor solution, the procedure ofExample 11 was repeated, except that the amount of polyethylene glycolused as the association preventive was reduced from 10% by weight to 5%by weight. As a result, unlike Example 11 (amount of associationpreventive used: 10% by weight) wherein no association of lead wasobserved, in this example (amount of association preventive used: 5% byweight), the association of lead was significant. The large number ofdefects in a stripe form appearing on the microphotograph(magnification: 20×) of the thin layer of PZT shown in FIG. 16demonstrates the association of lead. Regarding the prevention of theassociation of Pb, the description in Example 8 is true of this example.

Example 13

[0150] The procedure of Example 11 was repeated, except that the PZTprecursor paste was coated using a spatula instead of the doctor blade.As a result, as with Example 11, a crack-free, dense, thin layer of PZT(layer thickness 3 μm) was formed. The crystal structure of the thinlayer of PZT was examined by X-ray diffractometry and found to beconstituted by a single phase of PZT.

Example 14

[0151] The procedure of Example 11 was repeated, except that diethyleneglycol was used instead of polyethylene glycol as the associationpreventive. As a result, as with Example 11, a dense, thin layer of PZT(layer thickness 3 μm) free from precipitation of a material in a stripeform and a crack was formed. This demonstrates that, as with use ofpolyethylene glycol (Example 11), use of diethylene glycol as theassociation preventive results in the preparation of a highly stableferroelectric precursor and, in addition, coating of the precursor ontoa substrate followed by firing can provide a ferroelectric element whichis dense and possesses excellent piezoelectric properties.

Example 15

[0152] The procedure of Example 11 was repeated, except that glycerinwas used instead of polyethylene glycol as the association preventive.As a result, as with Example 11, a crack-free, dense, thin layer of PZT(layer thickness 3 μm) free from precipitation of a material in a stripeform was formed. This demonstrates that, as with use of polyethyleneglycol (Example 11), use of glycerin as the association preventiveresults in the preparation of a highly stable ferroelectric precursorand, in addition, coating of the precursor onto a substrate followed byfiring can provide a ferroelectric element which is dense and possessesexcellent piezoelectric properties.

Example 16

[0153] This example demonstrates the formation of a composite thin layerof PZT.

[0154] A first thin layer of PZT (substrate layer) was first preparedaccording to the following procedure. A solution of 0.00682 mol of leadnitrate in water, a solution of 0.00273 mol of zirconium oxychloride inwater, and a solution of 0.05 mol of potassium hydroxide in water weremixed together. A titanium substrate was immersed in the mixed solution,followed by hydrothermal treatment at 150° C. for 24 hr. As a result, acrystal nucleus of PZT was produced on the surface of the titaniumsubstrate. Separately, a solution of 0.00682 mol of lead nitrate inwater, a solution of 0.00273 mol of zirconium oxychloride in water, asolution of 0.00252 mol of titanium tetrachloride in water, and asolution of 0.05 mol of potassium hydroxide in water were mixedtogether, and the titanium substrate with a PZT crystal nucleus preparedin the above step was then immersed in the resultant mixed solution,followed by hydrothermal treatment at 120° C. for 48 hr. Thus, a firstthin layer of PZT having an average particle diameter of 5 μm and a verypoor surface smoothness was obtained.

[0155] Subsequently, on the first thin layer of PZT, formed on thetitanium substrate, was formed a second thin layer of PZT from anaqueous PZT precursor solution by the sol-gel process according to thefollowing procedure.

[0156] At the outset, 0.014 mol of titanium tetraisopropoxide wasdissolved in 30 g of a 3 N aqueous nitric acid solution to prepare anaqueous titanium solution. Separately, 0.016 mol of zirconium oxynitratewas dissolved in 20 g of pure water to prepare an aqueous zirconiumsolution. Further, 0.03 mol of lead nitrate was dissolved in 20 g ofpure water to prepare an aqueous lead solution. In this connection, itshould be noted that the ratio of Zr to Ti in PZT is closely related tothe ferroelectricity and this ratio is determined by the molar ratio ofthe corresponding metal salts. Therefore, the aqueous solutions ofrespective metal salts were prepared so that mixing was performed in adesired molar ratio.

[0157] The aqueous titanium solution and the aqueous zirconium solutionwere thoroughly mixed together at room temperature while stirring, andthe aqueous lead solution was then added to and mixed with the mixture.The present inventors have found that in practicing the presentinvention, the addition of aqueous metal salt solutions in the aboveorder is preferred. The resultant mixture was thoroughly stirred at roomtemperature and then heated under reflux at a temperature of 100° C. orabove to conduct hydrolysis. Metal ions contained in each aqueous metalsalt solution, when the aqueous solutions were mixed together at roomtemperature, merely formed metal hydroxides corresponding to respectivemetal ions, and the degree of bonding among the metal ions was small.Subsequent reflux and hydrolysis, however, permitted the metal ions,that is, titanium, zirconium, and lead ions, to be bonded to one anotherthrough oxygen. In this case, a Pb—O—Ti—O—Zr bond was formed to bringthe system to a sol. In the present invention, the aqueous solution inthis sol stage is an aqueous PZT precursor solution referred to in thepresent invention.

[0158] The temperature of the aqueous precursor solution was returned toroom temperature, water and a small amount of hydroxypropyl cellulosewere added dropwise thereto, followed by thorough mixing with stirring.In this case, the purpose of adding hydroxypropyl cellulose is toincrease the viscosity of the aqueous PZT precursor solution havingrelatively low viscosity and to enable the formation of a coating havinglarge thickness in the coating of the aqueous PZT precursor solution onthe substrate. Not only has hydroxypropyl cellulose the effect ofincreasing the viscosity, but also the effect of strengthening thePb—O—Ti—O—Zr bond and the effect of reducing the distance among theions. This can lower the firing temperature in the layer formation andin addition can improve the crystallinity of the layer. The aqueous PZTprecursor solution after the addition of hydroxypropyl cellulose was atransparent, homogeneous solution which was neither cloudy nor containedany precipitate.

[0159] The aqueous PZT precursor solution thus prepared was spin-coatedonto the substrate with the first thin layer of PZT formed above. Inthis case, the aqueous PZT precursor solution coated on the substratewith the first thin layer of PZT is in the uncrystallized state.Therefore, the coating was dried at 150° C. for 10 min and once fired ata temperature of 500° C. or above to decompose hydroxypropyl celluloseas the additive.

[0160] After the completion of the firing for the decomposition ofhydroxypropyl cellulose, the substrate was fired at 650° C. for one hr,thereby forming a second PZT structure having significantly enhancedsurface smoothness. Thus, a more dense, composite thin layer of PZT wasobtained which had no boundary between the first thin layer and thesecond thin layer.

[0161] In the production of an ink jet head, the thin layer of PZT,which has been formed through the above series of steps, has goodadhesion to a diaphragm and an electrode to be jointed to apiezoelectric element comprising the thin layer of PZT, so that they canbe integrally bonded without the aid of any adhesive. Use of thehydrothermal synthesis with the sol-gel process can bring the firingtemperature to a relatively lower value than that in the case of theconventional solid phase process. Therefore, the range of selection ofthe diaphragm as the substrate can be broadened.

Example 17

[0162] The procedure of Example 16 was repeated, except that the secondthin layer of PZT was formed according to the following procedure.

[0163] 0.014 mol of titanium tetrachloride was dissolved in aqueousammonia, and the resultant titanium hydroxide was collected byfiltration and washed. The collected titanium hydroxide was dissolved ina 3 N aqueous nitric acid solution to prepare an aqueous titaniumsolution. Separately, 0.016 mol of zirconium oxynitrate was dissolved in20 g of pure water to prepare an aqueous zirconium solution. Further,0.03 mol of lead nitrate was dissolved in 20 g of pure water to preparean aqueous lead solution.

[0164] The aqueous titanium solution and the aqueous zirconium solutionwere thoroughly mixed together at room temperature while stirring. Theaqueous lead solution was added to and mixed with the mixture. The mixedsolution was thoroughly stirred at room temperature and heated underreflux at a temperature of 100° C. or above to conduct hydrolysis. Thetemperature of the resultant mixed solution after the hydrolysis wasreturned to room temperature, water and a minor amount of hydroxypropylcellulose were added dropwise thereto, followed by thorough mixing withstirring. As a result, a homogeneous PZT solution was obtained.

[0165] The PZT solution thus prepared was spin-coated onto the titaniumsubstrate with the first thin layer of PZT formed above. The coating wasdried at 150° C. for 10 min and once fired at a temperature of 500° C.or above to decompose hydroxypropyl cellulose as the additive. Finally,firing was performed at 650° C. for one hr, thus completing theformation of a second thin layer of PZT.

[0166] As with the composite thin layer of PZT prepared in Example 16,the composite thin layer of PZT prepared in this example wassatisfactory.

Industrial Applicability

[0167] According to the present invention, aqueous solutions ofrespective metal oxides constituting a ferroelectric material areprepared and mixed together to prepare a mixed solution as aferroelectric precursor, and additives characteristic of the presentinvention, such as a thickener and an association preventive, are added.This constitution can offer many advantages unexpected from the priorart. For example, according to the present invention, the use of theaqueous metal oxide solution as the ferroelectric precursor isadvantageous in that handling and preparation are easy and, in addition,the storage stability of the starting compound solution is excellent,addition of an association preventive can enhance the stability of theprecursor, a ferroelectric element can be easily produced at low costand can be formed as a thin layer, and, since particles on the surfaceof the thin layer are fine and dense, excellent surface smoothness canbe realized.

[0168] Further, according to the present invention, addition of aferroelectric powder, an organic binder and the like to prepare aprecursor paste followed by the formation of a thin layer of aferroelectric can prevent the creation of defects, such as precipitationof a material in a stripe form, on the surface of the thin layer of aferroelectric, can provide a dense, homogeneous, thin layer of aferroelectric, ensuring excellent piezoelectric properties. In addition,this can prevent the creation of cracking on the surface of the thinlayer.

[0169] Furthermore, according to the present invention, stacking of asecond thin layer of a ferroelectric prepared by the sol-gel process ona first thin layer of a ferroelectric prepared by hydrothermal synthesisenables the thickness of the ferroelectric element to be increased whilemaintaining the features of the ferroelectric element of the presentinvention and, in addition, can offer good adhesion to the underlyingsubstrate.

[0170] In addition, application of the ferroelectric element of thepresent invention as a piezoelectric element to a piezoelectric ink jethead can, of course, offer excellent piezoelectric properties and,further, permits an electrode to be easily formed on the piezoelectricelement without creating any defect.

What is claimed is:
 1. A ferroelectric element comprising aferroelectric material containing at least two metals, saidferroelectric element having been produced by a process including asol-gel process in the presence of a thickener and/or an associationpreventive from aqueous solutions of respective salts of the metals. 2.The ferroelectric element according to claim 1, wherein the thickenercomprises a water-soluble polymeric material which, when heated to atemperature above a predetermined temperature at the time of theformation of the ferroelectric element, can be heat-decomposed.
 3. Theferroelectric element according to claim 2, wherein the thickenercomprises at least one compound selected from the group consisting ofhydroxyalkyl cellulose, polyethylene oxide, and polyvinyl alcohol. 4.The ferroelectric element according to claim 1, wherein the associationpreventive comprises a water-soluble polyhydric alcohol.
 5. Theferroelectric element according to claim 4, wherein the polyhydricalcohol comprises at least one compound selected from the groupconsisting of diethylene glycol, polyethylene glycol, and glycerin. 6.The ferroelectric element according to any one of claims 1 to 5, whereina powder of another ferroelectric material having a crystal structureidentical or similar to the ferroelectric material is present inaddition to the thickener and/or the association preventive.
 7. Theferroelectric element according to any one of claims 1 to 6, wherein theferroelectric material is a ceramic having a perovskite structure andone of the metals is lead.
 8. The ferroelectric element according toclaim 7, wherein the salt of lead is lead nitrate or lead acetate. 9.The ferroelectric element according to claim 7, wherein the ceramic islead zirconate titanate.
 10. The ferroelectric element according to anyone of claims 1 to 9, which is in the form of a thin layer.
 11. Theferroelectric element according to any one of claims 1 to 10, whichfurther comprises a ferroelectric substrate layer having a crystalstructure identical or similar to the ferroelectric material and hasbeen formed by hydrothermal synthesis from aqueous solutions of metalsnecessary for the formation of the substrate layer.
 12. Theferroelectric element according to claim 11, wherein the particlediameter of the ferroelectric material constituting the substrate layeris larger than that of the ferroelectric material constituting theoverlying layer.
 13. A process for producing a ferroelectric elementcomprising a ferroelectric material containing at least two metals, saidprocess comprising the steps of: mixing aqueous solutions of respectivesalts of the metals together to prepare an aqueous ferroelectricprecursor solution; adding a thickener and/or an association preventiveto the aqueous ferroelectric precursor solution and coating theresultant-solution onto a substrate; and drying and firing the coatingto crystallize the ferroelectric material.
 14. The process according toclaim 13, wherein the thickener is a water-soluble polymeric materialand can be heat-decomposed in the step of crystallizing theferroelectric material.
 15. The process according to claim 13, whereinthe association preventive comprises a water-soluble polyhydric alcohol.16. The process according to any one of claims 13 to 15, wherein apowder of another ferroelectric material having a crystal structureidentical or similar to the ferroelectric material is further added inthe step of adding the thickener and/or the association preventive. 17.The process according to any one of claims 13 to 16, which furthercomprises the step of adding the ferroelectric precursor in the form ofa paste derived from the aqueous ferroelectric precursor solution. 18.The process according to any one of claims 13 to 17, which furthercomprises the step of forming on the substrate a substrate layer, havinga crystal structure identical or similar to the ferroelectric material,from aqueous solutions of metals necessary for the formation of thesubstrate layer by hydrothermal synthesis.
 19. The process according toany one of claims 13 to 18, wherein the ferroelectric material is aceramic having a perovskite structure and one of the metals is lead. 20.The process according to claim 19, wherein the ceramic is lead zirconatetitanate.
 21. A ferroelectric precursor for use as a starting materialin the production of a ferroelectric material comprising at least twometals by a process including a sol-gel process, said ferroelectricprecursor comprising aqueous solutions of respective salts of the metalsand containing a thickener and/or an association preventive.
 22. Theferroelectric precursor according to claim 21, which further comprises apowder of another ferroelectric material having a crystal structureidentical or similar to the ferroelectric material.
 23. An ink jet headcomprising a plurality of nozzles for ejecting an ink, ink chambers, forpassage and pressurization of the ink, communicating with the nozzles,and pressing means for creating a change in volume of the ink in the inkchamber to eject the ink through the nozzles, the pressing meanscomprising a ferroelectric element as a piezoelectric element, saidferroelectric element comprising a ferroelectric material containing atleast two metals and having been produced by a process including asol-gel process in the presence of a thickener and/or an associationpreventive from a aqueous solutions of respective salts of the metals.24. The ink jet head according to claim 23, wherein the ferroelectricmaterial is a ceramic having a perovskite structure and one of themetals is lead.
 25. The ink jet head according to claim 24, wherein theceramic is lead zirconate titanate.